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Microtensile strain on the corrosion performance
of diamond-like carbon coating
Seung-Hwan Lee,
1
Jung-Gu Kim,
1
Heon-Woong Choi,
2
Kwang-Ryeol Lee
2
1
Department of Advanced Materials Engineering, Sungkyunkwan University, 300 Chunchun-Dong,
Jangan-Gu, Suwon 440-746, Korea
2
Future Technology Research Division, Korea Institute of Science and Technology, P.O. Box 131,
Cheongryang, Sungbuk-Gu, Seoul 130-650, Korea
Received 10 March 2007; revised 13 June 2007; accepted 26 June 2007
Published online 26 September 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31597
Abstract: Hydrogenated diamond-like carbon films (a-
C:H DLC) were deposited on STS 304 substrates for the
fabrication of vascular stents by means of the r.f. plasma-
assisted chemical vapor deposition technique. This study
provides reliable and quantitative data for the assessment
of the effect of strain on the corrosion performance of
DLC-coated systems in the simulated body fluid obtained
through electrochemical techniques (potentiodynamic
polarization test and electrochemical impedance spectros-
copy) and surface analysis (scanning electron microscopy).
The electrolyte used in this test was 0.89% NaCl solution
at pH 7.4 and 378C. It was found that the corrosion resist-
ance of the plastically deformed DLC coating was insuffi-
cient for use as a protective film in a corrosive body envi-
ronment. This is due to the increase in the delamination
area and degradation of the substrate’s corrosion proper-
ties with increasing tensile deformation. Ó2007 Wiley
Periodicals, Inc. J Biomed Mater Res 85A: 808–814, 2008
Key words: diamond-like carbon; stent; microtensile test;
electrochemical impedance spectroscopy; potentiodynamic
polarization test
INTRODUCTION
Using protective films to coating implants, in order
to reduce their level of corrosion and wear, may
extend their lifetime to the benefit of the patients. Dia-
mond-like carbon (DLC), which is characterized by
chemical inertness, corrosion, and wear resistance,
appears to be an ideal material for such purposes.
1
Because of its bio- and hemocompatible nature,
2–5
there is a growing interest in the application of DLC
to orthopedic and blood contacting implants.
6
Today,
there are two main areas of application of DLC in bio-
logical applications, namely in blood contacting
implants such as heart valves and stents, and in load
bearing joints to reduce the level of wear. The load-
bearing properties of the implants are mainly con-
trolled by their bulk properties, whereas the interac-
tion with the surrounding tissue is governed by the
implant surface.
7
Neither natural diamond nor DLC
coatings cause tissue reactions, and the corrosion of
implants can be decreased significantly by using such
a coating.
8
However, the high hardness, intrinsic
stresses, and poor adhesion of these materials limit
their area of application. These negative effects are
especially pronounced when the coatings are applied
to relatively soft substrates such as steels. Such prob-
lems have been reduced using a multilayer design, in
which metal and ceramic layers are used to increase
the strength of adhesion, relax the compressive stress
of the DLC film, and increase the load support capa-
bility.
9,10
One or more interim layers are introduced to
improve the adhesion of the DLC to the metallic sub-
strate. Moreover, the adhesion of a DLC film with an
intermediate layer of a-SiC
x
or TiC was found to be
significantly improved.
4,11–13
However, a report on
DLC coatings in total hip arthroplasty (THA) showed
delamination and brittling under in vivo conditions,
and it was observed that the spallation of the DLC
coating during the experiment simulated the expan-
sion of the vascular stent. The DLC coating on stain-
less steel used to prevent the elution of Ni and Cr
should survive the plastic deformation of the sub-
strate.
14,15
In this paper, we focused on evaluating the
Correspondence to: J.-G. Kim; e-mail: kimjg@skku.ac.kr
Contract grant sponsor: Center for Nanostructured
Materials Technology (21st Century Frontier R&D Pro-
grams of the Ministry of Science and Technology, Korea);
contract grant number: 06K1501-01610
'2007 Wiley Periodicals, Inc.
variation of the corrosion performance of the DLC
coating with strain (maximum 4%) during the expan-
sion of the vascular stent through electrochemical
techniques.
MATERIALS AND METHODS
A 304 stainless steel with a thickness of 0.2 mm was
used as the substrate material, and was electrochemically
polished to obtain an rms surface roughness of less than
0.1 lm. Before deposition, the substrates were precleaned
using an argon plasma for 60 min at a bias voltage of
900 V. A Si interlayer with a thickness of 98 nm was
deposited onto the substrate prior to the DLC coating, in
order to improve the adhesion between the coating and
substrate. The DLC films were deposited by the radio fre-
quency plasma-assisted chemical vapor deposition (r.f.
PACVD) method using benzene as the precursor gas. The
residual compressive stress and hardness of the DLC films
in this work were 0.9 and 10 GPa, respectively. The
detailed deposition conditions are given in Table I.
Electrochemical techniques were used to evaluate the
influence of the microtensile strain on the corrosion per-
formance. The potentiodynamic polarization test was per-
formed with an EG&G Princeton Applied Research model
273A potentiostat. The potentiodynamic polarization test
was carried out in a 0.89% NaCl solution at pH 7.4 and
378C, which was thoroughly deaerated by bubbling high
purity nitrogen gas for 0.5 h prior to the immersion of the
specimen and continuously purged during the test. The
exposed specimen area was 0.25 cm
2
. A saturated calomel
electrode and pure graphite were used for the reference
and counter electrodes, respectively. Prior to the potentio-
dynamic polarization test, the specimens were kept in the
solution for 3 h to obtain a stabilized open-circuit poten-
tial. The potential of the electrode was swept at a rate of
0.166 mV/s from the bottom potential of 250 mV versus
E
corr
to the top potential of 1600 mV. The porosity and
protective efficiency of the DLC coating were estimated
using the potentiodynamic polarization, and the delamina-
tion area was estimated using the electrochemical imped-
ance spectroscopy (EIS) test. Matthes et al.
16
established an
empirical equation to estimate the porosity of coatings:
P¼RpmðsubstrateÞ
RpðcoatingsubstrateÞ
310jDEcorrbajð1Þ
where Pis the total coating porosity, R
pm
the polarization
resistance of the substrate, and R
p
the measured polariza-
tion resistance of the coated system. DE
corr
is the potential
difference between the corrosion potentials of the coated
steel and the bare substrate, and b
a
the anodic Tafel slope
for the substrate. Also, the protective efficiency of the coat-
ing was determined from the polarization curve by means
of Eq. (2):
Pi¼100 ð1icorr=io
corrÞð2Þ
where i
corr
and i
o
corr
indicate the corrosion current densities
in the presence and absence of the coating, respectively.
17
EIS is a nondestructive testing method frequently used
for assessing the protective performance of coatings. A
Zahner IM6e system using a commercial software
(THALES) program for AC measurement was used to
obtain the EIS data. The impedance measurements were
performed by applying a sinusoidal wave with an ampli-
tude of 10 mV to the working electrode, at frequencies
ranging from 10 kHz to 10 mHz. The impedance diagrams
were interpreted on the basis of the equivalent circuit
using the THALES fitting program. The delamination area
of the coatings exposed to the electrolyte was determined
using EIS. Thus, the extent of the delamination area could
be determined from the experimental values of the pore
resistance obtained from the impedance diagrams on the
basis of the equivalent circuit. The pore resistance of the
coating is related to the delamination area, that is, the pore
resistance decreases as the delaminated area increases.
Therefore, the delamination area was calculated by means
of the following equations.
Ad¼Ro
pore=Rpore ð3Þ
Ro
pore ¼qdðohm cm2Þð4Þ
where R
o
pore
is the characteristic value for the corrosion
reaction at the solution–coating interface, dis the coating
thickness, and qis the coating resistivity.
Scanning electron microscopy (SEM) was used to exam-
ine the delamination and spallation of the coatings after
the EIS test, and 5003and 20003SEM images were
TABLE I
Main Deposition Conditions
Deposition method r.f. PACVD (13.56 MHz)
Base pressure 2.0 310
5
Pa
Interlayer Silicon interlayer
Precleaning time 60 min (Ar sputtering)
DLC precursor gas C
6
H
6
Deposition pressure 2.0 310
2
Pa
Bias voltage (deposion/precleaning) 400 V/900 V
Si interlayer thickness 98 nm
Film thickness 1 lm
Figure 1. Potentiodynamic polarization curves in deaer-
ated 0.89% NaCl solution at 378C (pH 7.4).
MICROTENSILE STRAIN ON CORROSION PERFORMANCE OF DIAMOND-LIKE CARBON COATING 809
Journal of Biomedical Materials Research Part A
obtained for this purpose. A SE detector was used and the
acceleration voltage was 15 keV.
RESULTS AND DISSCUSION
The protective ability of the coating was investi-
gated using the potentiodynamic polarization test.
The polarization curves of the DLC coatings and
substrate in the simulated body fluid are shown in
Figure 1. The measured potentiodynamic polariza-
tion test results, such as the corrosion potential
(E
corr
), corrosion current density (i
corr
), porosity (P),
and protective efficiency (P
i
) are shown in Table II.
The corrosion current densities were 6.557 nA/cm
2
for the substrate, 0.312 nA/cm
2
for the 0%-strained
coated system, 1.126 nA/cm
2
for the 2%-strained
coated system, and 4.720 nA/cm
2
for the 4%-
strained coated system. The corrosion current den-
sities for the coated systems were lower than that for
the substrate. This means that the coating with fewer
pores makes the substrate more passive than the
coating with a larger number of pores. In this solu-
tion, substrate and DLC coatings exhibited passive
behavior. However, as the strain increased, DLC
coatings suffered active-to-passive transitions and
the pitting potential also decreased, which were in-
dicative of active dissolution or incomplete passivity.
This causes a high local current density and induces
high metal dissolution at anode. Another possible
mechanism may involve a periodic galvanic interac-
tion between DLC coating and the uncovered stain-
less steel. As a result of an electrochemical potential
difference between the substrate and DLC film, a
small electrical current is generated between the an-
odic metal and the cathodic film. The relatively
small area of the substrate metal surface compared
to the large surface area of DLC film results in an
unfavorable anode-to-cathode ratio. These pores can
weaken the interfacial material and provide a path
for metallic ions and corrosive agents. The lower the
calculated porosity, the lower the corrosion current
density. The protective efficiency of the coating
decreases as the tensile deformation proceeds, and is
appreciable after plastic deformation. The 0%-
strained coating shows the best protective efficiency
of 95.25%, and this result is closely related to the po-
TABLE II
Results of Potentiodynamic Tests
E
corr
(mV) i
corr
(nA/cm
2
)b
a
(V/decade) b
c
(V/decade) R
p
(10
3
Ocm
2
)
Protective
Efficiency (%) Porosity
Sub 33.9 6.557 0.2981 0.0693 3726.2217 – –
0% 32.55 0.312 0.2284 0.1425 122441.5576 95.25 0.0301
2% 29.86 1.126 0.4851 0.1137 35566.7122 82.83 0.1015
4% 69.32 4.720 0.1000 0.1041 4698.2691 28.02 0.6033
Figure 2. Nyquist plots for (a) 0%, (b) 2%, and (c) 4%-
strained specimens.
810 LEE ET AL.
Journal of Biomedical Materials Research Part A
rosity. The protective efficiency increases as the po-
rosity decreases and, consequently, the best corro-
sion resistance and durability are obtained for the
coating with fewer pores and lower strain.
Nyquist plots of DLC-coated specimens with dif-
ferent strain are shown in Figure 2. The interpretation
of the EIS measurements is usually done by fitting
the impedance data to an equivalent circuit, which is
representative of the physical processes taking place
in the system under investigation. The electrochemi-
cal response during the EIS measurements for the
DLC coatings was best simulated with the equivalent
circuit, as shown in Figure 3. The results of EIS meas-
urements were given in Table 3. The equivalent cir-
cuit consists of the following elements: R
s
is the solu-
tion resistance of the test electrolyte between the
working electrode and the reference electrode, and
C
coat
is the coating capacitance generated by the
dielectric properties of the coating. C
coat
corresponds
to the dielectric strength of the coating and the water
absorption by the coating. Higher values indicate
higher dielectric strength or higher water content.
R
pore
is the electrical resistance resulting from the for-
mation of ionic conduction paths through the pores
in the coating. Higher values indicate higher resist-
ance to penetration of corrosive species. C
dl
is the ca-
pacitance generated by the electric double layer at
the water/substrate interface. An appreciable C
dl
value indicates that water is present at the substrate.
Higher values of C
dl
indicate a greater wetted area of
substrate. R
ct
is the charge-transfer resistance of the
substrate to corrosion. Higher values indicate lower
rates of corrosion. Constant phase elements (CPEs)
are used in the data fitting, to allow for depressed
semicircles. The capacitances are replaced with CPEs
in order to improve the quality of the fit. The CPEs
are, in fact, a general expression for many circuit ele-
ments. In this paper, C
coat
and C
dl
are replaced with
CPE1 and CPE2, respectively.
The variations of the capacitance are indicated in
Figure 4(a,b). In the case of 4% strain, it is shown that
the capacitance increases significantly with increasing
immersion time, whereas the 0% and 2%-strained
coatings show only a slight increase or a small varia-
tion. This is because the coating with the 4% plastic
deformation has more pores and allows more water
to be adsorbed by the substrate. As shown in Figure
4(c), the pore resistance (R
pore
) of the coatings
decreases gradually with increasing immersion time.
The coating may swell, and the number and size of
the pores increase. The decrease in the pore resist-
ance of the coating corresponds to the occurrence of
water saturation, as depicted by the increase of the
Figure 3. Equivalent circuit for the DLC film systems.
(WE: working electrode, RE: reference electrode).
TABLE III
Results of Electrochemical Impedance Spectroscopy Measurements
Exposure Time R
s
(Ocm
2
)
CPE1
R
pore
(10
3
Ocm
2
)
CPE2
R
ct
(10
3
Ocm
2
)
C
coat
(10
9
F/cm
2
)n(0–1) C
dl
(10
9
F/cm
2
)n(0–1)
24 h Strain 0% 202.6 2.355 0.9094 6.063 2.171 1 3621
Strain 2% 5.32 29.18 0.8774 2.071 19.39 1 221.3
Strain 4% 4.383 394.7 1 1.355 472.9 0.7311 52.57
72 h Strain 0% 89.79 1.871 0.9103 2.047 4.195 0.9188 3166
Strain 2% 13.01 29.59 0.8822 1.326 59.52 0.9304 146.1
Strain 4% 1.927 152.5 0.6917 0.2207 415.5 1 17.67
120 h Strain 0% 100.8 2.04 0.8834 2.613 7.85 0.2753 1008
Strain 2% 10.06 36.07 0.8367 1.461 100.8 0.8321 118.3
Strain 4% 2.825 181.3 1 0.1842 575.9 0.666 8.505
168 h Strain 0% 52.63 2.151 0.8842 2.657 6.019 0.194 772
Strain 2% 33.12 47.85 0.8614 1.313 120.7 0.7967 109.6
Strain 4% 1.037 445.3 0.9074 0.1961 608.9 0.719 6.035
216 h Strain 0% 1.609 2.33 0.8813 2.73 7.44 0.937 726.1
Strain 2% 10.03 52.29 0.8238 0.8094 193.9 0.799 100.8
Strain 4% 9.997 654.6 0.9074 0.1047 778 0.712 5.559
MICROTENSILE STRAIN ON CORROSION PERFORMANCE OF DIAMOND-LIKE CARBON COATING 811
Journal of Biomedical Materials Research Part A
coating capacitance. Also, the pore resistance of the
coating decreases as more ions and water reach
the coating surface, causing an increased electro-
chemical reaction. According to Figure 4(d), the charge
transfer resistance (R
ct
) decreased with increasing
immersion time. This means that water and ions would
have gradually migrated to the substrate surface. The
charge-transfer resistance of the less strained coating
is higher than that of the more strained one. The plas-
tic deformation of the specimen resulted in the film
rupture process, which is consistent with the reduc-
tion of the resistance.
Also, the results obtained from the EIS measure-
ments are usually used to monitor the change of the
delamination area (A
d
). Figure 5 shows the delamina-
tion areas of the three kinds of DLC coatings. The
delamination area of the coating with 4% strain
increases significantly with increasing immersion
time, as shown in Figure 5. On the other hand, the
delamination areas of the coatings with 0 and 2%
strain show only a slight increase or become stabilized
with increasing immersion time. The delamination
area is affected by the penetration of water through
the porous coating, because water saturation in the
coating/substrate interface leads to delamination and
blistering. Consequently, the delamination area is
much lower in the coating with lower strain than in
that with higher strain. It is clear that the delamina-
tion area is closely related to the penetration of water
through the pores and defects. Structural defects,
such as pinholes, pores, and cracks, act as channels
for the corrosion of the substrate. The porosity of the
coating is the main cause of coating delamination.
Figure 4. (a) Coating capacitance, (b) double-layer capacitance, (c) pore resistance, (d) charge-transfer resistance.
Figure 5. Delamination area as a function of immersion
time.
812 LEE ET AL.
Journal of Biomedical Materials Research Part A
After the completion of the EIS test, the morphol-
ogy and corrosion features of the DLC-coated sys-
tems with 0, 2, and 4% micro-tensile strain were
inspected by SEM and the resulting micrographs are
shown in Figure 6. Choi et al.
15
show that the yield
point of the specimen is between 3 and 4% strain in
the force–strain curve. When the stainless steel sub-
strate was elastically deformed, there were no appre-
ciable defects in the film. However, after the stain-
less steel substrate was plastically deformed (4%-
strained), crack propagation occurred along the slip
directions. This means that the tensile strain of the
substrate affects the integrity of coating; thus, the
plastic deformation is closely related with the protec-
tive ability of coating. Since the elution of Ni and Cr
through spallation has a deleterious effect on the
human body,
15
the coating system with higher stress
corrosion resistance is more adequate for protective
films in the body environments.
CONCLUSIONS
Using electrochemical techniques and surface anal-
ysis, the corrosion behavior of DLC coatings on 304
stainless steel substrates subjected to different levels
of tensile strain was investigated.
Figure 6. SEM microstructures of the specimen surface after imposing tensile strains of (a) 0%, (b) 2%, and (c) 4%-
strained.
MICROTENSILE STRAIN ON CORROSION PERFORMANCE OF DIAMOND-LIKE CARBON COATING 813
Journal of Biomedical Materials Research Part A
1. The DLC coatings with lower strain showed a
lower corrosion current density and porosity
than the plastically deformed coating, indicating
that the former had better corrosion resistance.
2. Decreasing the level of stain reduced the de-
lamination area in the DLC coatings. Also, the
charge-transfer resistance values of the coated
system increased as the stain decreased.
3. When the stainless steel substrate was plasti-
cally deformed, an increase in the delamination
area and spallation behavior of the coating was
observed in the EIS and SEM analyses, which
means that plastic deformation decreases the
protective ability of the DLC coating.
4. Consequently, the corrosion performance of
DLC coatings decreased as the strain increased,
which was caused by galvanic coupling
between DLC coating and the uncovered stain-
less steel through the film rupture.
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Journal of Biomedical Materials Research Part A
